The risk of explosion during the transport and storage of liquid fuels

. When transporting and storing liquid fuels, explosive atmospheres can form which, under certain conditions, can be initiated with different sources, of which static electricity has proven to be the source of ignition in many cases of fires and explosions that have occurred. As a result of the environmental regulatory requirements to reduce the noxes emitted by diesel engines, the oil industry introduced into production fuels with ultra-low sulfur content (ULSD Ultra Low Sulfur Diesel), clean burning diesel, and low sulfur content (LSD Low Sulfur Diesel). The new types of fuels present a greater risk of explosion than fuels with a high sulfur content because they have other explosive characteristics and are electrostatically charged much more easily. The paper presents some aspects regarding the risk of explosions during the transport and storage of liquid fuels with the highlighting of additional risks in the case of ULSD Also, the paper highlights the factors that must be taken into account when establishing the technical and organizational measures that must be taken to prevent ignition and that must be implemented in the work procedures of manufacturers, suppliers, transporters and users of ULSD.


Introduction
Many of the operations with flammable liquids produce flammable atmospheres by evaporation of the handled liquid.The flash point gives a rough indication of the minimum liquid surface temperature that is sufficient to produce a flammable atmosphere in the process.
However, because of the uncertainty involved in measuring the flash point, the differences between the flash point test conditions and those in a real industrial situation, and because of the difficulty of establishing the surface (rather than bulk) temperature of the liquid, it is prudent to it is assumed that a flammable atmosphere could exist even when the temperature of the liquid is below a safety limit of the flash point.
The margin of safety depends on the level of uncertainty about the temperature, liquid composition, etc.For well-controlled conditions a limit of 5 ºC is required for pure liquids and a minimum of 11 ºC for mixtures [1].
If the containers are exposed to direct sunlight and the temperature of the liquids is not monitored, it must be assumed that a flammable atmosphere could occur when handling liquids with flash points up to 60 °C.In areas with high ambient temperature and strong sunlight, flammable atmospheres can occur even with liquids that have flash points above 60 °C.
If a liquid is handled at a temperature that is well above its flash point, then the saturated vapors can give rise to an over-enriched (thus non-flammable) atmosphere.However, the actual atmosphere above the liquid may not be saturated (eg due to ventilation, venting) and thus be flammable.This is why it is necessary to assume that the atmosphere may be flammable, unless the contrary can be proven with certainty.Consequently, for liquids with a low flash point, the presence of an over-enriched atmosphere cannot be relied upon as the sole control measure.
Under certain conditions, the flammable atmosphere is not due to the handled liquid but due to volatile liquid or vapor residues from previous operations performed with the same equipment or from operations in the vicinity.Residual vapors can appear, for example, during alternate filling with another liquid, for example when a liquid with a high flash point (eg diesel) is loaded into a tank that previously contained a liquid with a low flash point (eg petrol).A large proportion of tanker fires are associated with switch loading.
If we refer to the sources of ignition of the explosive atmosphere, it is observed that most accidents given in specialized literature cite static electricity as the most likely source of ignition.
In Germany, between 1992 and 1995, more than 30 incidents of ignition of gasoline/air mixtures during vehicle refueling were reported.Upon investigation, it was thought that electrostatic charging was most likely responsible for most of them [2] On July 17, 2007, an explosion and fire was reported at the Barton Solvents Wichita facility in Valley Center, Kansas, at approximately 9 a.m.The incident left the tank completely destroyed.Eleven residents and one firefighter had to receive medical treatment.This incident was investigated by the US Chemical Safety and Hazard Investigation Board (CSB).It was determined that the initial explosion occurred inside an above-ground vertical storage tank while it was being filled with Paint Manufacturers and Painters (VM&P) naphtha, which is a National Fire Protection Association Class IB flammable liquid.(NFPA) that can produce flammable vapor-air mixtures inside tanks and is also capable of accumulating dangerous levels of static charge due to its low electrical conductivity [3] In 2008, a flash fire was reported to have occurred at a process industry company's production site while filling 200 L drum drums with isopropyl acetate [4].This was also attributed to electrostatic causes.
In our country too, there have been several ignitions of flammable atmospheres due to electrostatic discharges.In 2020, there were two explosions at a refinery followed by fires at a loading ramp during the loading of diesel fuel with a low conductivity and low sulfur content (< 50 ppm sulfur) into the tanker [5,6].The investigation of the causes that led to the occurrence of the two events carried out by INCD-INSEMEX Petroşani led to the hypothesis that accumulated discharge of static electricity was the source of ignition of the explosive atmosphere due to the fact that low conductivity diesel fuel was loaded into the tank at high speed.Consequently, the Switch Loading, the process of loading diesel in a compartment that previously carried gasoline, favored the formation of the explosive mixture of gasoline vapors inside compartment.For an explosion to take place, there must be an explosive atmosphere and an efficient ignition source.When liquid fuels are transported and stored at temperatures higher than their flash point, explosive atmospheres can form which, under certain conditions, can be initiated by different sources, of which static electricity has proven to be the most frequent.The electrical conductivity of the fuel is an essential parameter for assessing the risk of charging/discharging static electricity.
If we analyze some Safety data sheets of ULSD issued by the manufacturing companies, we can see that there is no agreement between them regarding the point of flammability.The conductivity of the liquid is also rarely given.-ULSD at terminal load rack with conductivity additive: At least 50 pS/m.
In order to assess the risk of explosion at an installation, the explosion characteristics must be known, the flash point being essential for liquids.Also, if we consider the possibility of igniting the atmosphere through electrostatic discharges, an analysis of the process and the installation must be done regarding the possibility of the formation, accumulation and discharge of electrostatic charges both from the loaded fuel and from the parts of the installation.
The first step that must be taken is to correctly determine the explosion characteristics, essential being the flash point and conductivity of the liquid fuel, which must be listed in the Material Safety Data Sheets (MSDS).

Charging/discharging of static electricity
Electrostatic charging of liquid fuels is most commonly generated when they flow through pipes, hoses and filters or when agitated.Liquid fuels can also become charged if they are transferred to a container that is either already charged (for example, an electrostatically charged plastic container) or charged while containing the liquid (for example, when the outer surface of a plastic container containing a liquid is rubbed).Charge can build up in the liquid if it is electrostatically insulating or electrostatically isolated from ground.
Accumulation of charges to dangerous levels is mainly associated with liquids of low conductivity.These levels can also appear in liquids with medium conductivity, when the loading rate is high, for example high flow rates or agitation of certain suspensions.In practice, dangerous levels have not been encountered with highly conductive liquids, provided they are properly grounded.
The build-up of electrostatic charge on the surface of the liquid can give rise to electrostatic discharges that may be energetic enough to ignite flammable vapors, such as those that may be released into the headspace of a container.
Unless electrically insulating components (e.g.plastics) are used, the charge on the containment system is rapidly dissipated by earthing or bonding.However, if the solvent has a low conductivity (≤ 50 pS/m), the solvent charge only dissipates relatively slowly.
In this case, when the solvent collects in a tank, the accumulation of charge in the liquid can create high surface voltages and lead to strong descharges that can ignite solvent vapours.
With high liquid surface potentials, brush discharges can occur between the surface of the charged liquid and metallic parts of the tank structure (fig 4).
An ignition hazard can arise if isolated conductors or improperly bonded components are present in the tank.An example would be a metal can floating on a static accumulator and the side of a tank-truck compartment (fig 5).

3-Ground
The accumulation of charges in a liquid is determined by two opposite effects, charge generation and charge relaxation.
When a liquid flows through a pipe, a charge separation occurs creating a charge on the liquid.Turbulent flow generates a higher charge compared to laminar flow.For a singlephase liquid, the electric flow current generated in a long pipe is almost proportional to the velocity in the case of laminar flow and to the square of the velocity in the case of turbulent flow.Since flow in industrial plants is usually turbulent, only turbulent conditions will be considered.
If the liquid enters the pipe uncharged, the flow current, and thus the charge density carried by the liquid, will increase with the length of the pipe and will gradually approach a stationary value if the pipe is long enough.For liquids with low conductivity (especially saturated hydrocarbons), the stationary charge density value is:   v, (C/m 3 ) (1) where:  is the charge density for a pipe of infinite length v is the linear velocity of the liquid in the pipe (m/s) For the speed range from 1 m/s to 10 m/s the charge densities are between 5 and 50 C/m3.
For practical purposes the pipe may be considered to be of infinite length if L  v ;  0 / , (m) (2) where: L is the length of the pipe;  is the relaxation time of the liquid (in s);  is the relative permittivity of the liquid (approx. 2 for hydrocarbons); 0 is the permittivity of free space (0 =8.85 x 10 -12 F/m);  is the electrical conductivity of the liquid (S/m).This expression can be used to approximate the charge density of liquid coming out of a pipe, for example while filling a tank.
When liquids containing a non-homogenizable second phase, such as dispersed liquids or suspended solids, are pumped through pipelines, the load generation coefficient is much higher compared to the single-phase situation.
The relaxation of charges in a container of liquid is determined by its electrical conductivity.
Regarding relaxation time, the rules define relaxation time: time during which the electrostatic charge on a solid surface or in the bulk of a liquid or powder decays exponentially to 1/e (i.e. about 37 %) of its original value [1] NOTE For high charge generation with high resistivity liquids, relaxation may be hyperbolic rather than exponential.
In general, for liquid hydrocarbons, the relaxation time constant is approximated by the relation: τ = 18/ where: τ is the relaxation time (s);  is the electrical conductivity of the liquid în pS / m.For example, a liquid with a conductivity of 6 pS/m would have a relaxation time constant of 3 s.So a charge can be considered "virtually dissipated" after three time constants and "completely dissipated" after five time constants time.
In the transportation process of liquid fuels, they are often passed through a filter before being introduced into a receiving vessel or container.Leakage of liquid fuel through filters generates relatively high levels of electrostatic charge due to the increased surface area available for contact (fig 6).

Fig. 6. Separation of charges in a filter
It is desirable to dissipate the electrostatic charge from a flammable liquid before it enters a receiving vessel to reduce the potential hazard of electrostatic ignition.This is usually done by placing the filters as far upstream as possible from the receiving vessels so that the charge can be dissipated from the liquid in the grounded metal pipe downstream of the filter before the liquid is introduced into the vessel.
A filter can produce 10 to 200 times more charge than this product in the same system without filtration.In some cases, wire mesh can also improve load generation.There is no danger from this overcharging as long as the liquid is contained in the pipe; the absence of air will not allow any flammable mixture to ignite.In addition, the high load developed by the filter tends to decrease as the liquid continues down the line (fig 7).When the pore or screen size of the filter is greater than 300 microns (less than 50 mesh / in.), it is unlikely that dangerous levels of electrostatic charge will be generated in the filter / screen.Therefore, no specific provision for downstream relaxation is required.NOTE : Residence time is defined as : the length of time that a product remains in a grounded conductive delivery system from the point at which a charge is generated before it reaches the point of delivery, e.g. from the outlet of a pump, an inline filter or a microfilter to the inlet of a tank truck, a tank or marine vessel.As the filter pore size decreases, charge generation increases and can approach dangerous levels.When the pores or the size of the screen are less than 150 microns, the charge level is likely to generate dangerous tests.In these situations, a residence time (residence) of at least 30 seconds must be ensured between the filter or screen and the discharge point.A distinction must be made between relaxation time and residence time: relaxation time is calculated and residence time is a function of equipment/system configuration and operating parameters.[7] With pore sizes between 300 microns (50 mesh) and 150 microns (100 mesh), safe operations may be possible based on a facility risk assessment that considers factors such as materials handled and operating procedures in place.
Regardless of pore size, filters and wire screens must be cleaned or replaced if the pressure drop becomes excessive, as load generation increases when the filter and screen become partially obstructed.In practice, a pipe is inserted downstream of the filter long or wide enough to retain the liquid for a sufficient residence time to relax the load to a value considered safe.
The same result can be achieved by keeping the liquid in a relaxation chamber for the required time or by reducing the flow rate.A relaxation chamber must be operated full of liquid to avoid the possibility of sparking a flammable vapor space.
Ensuring at least 30 seconds dwell time (and preferably 100 seconds) after filtering is recommended as a design criterion for all new hardware, regardless of product or service.Applied to the filling of tanks, hoppers and containers, as well as to loading racks (racks), this design consideration will provide protection if the equipment is converted to another service or if the product is accidentally contaminated.It will also provide protection when the switch charging may occur.
The required residence time must not be greater than 100 seconds, unless the switch loading precautions or the initial filling rate limitation are not observed.Where conductivity cannot be checked a minimum dwell time of 100 seconds should be used.
However, from a technical point of view, the residence time requirement can be ignored for products whose conductivity is greater than 50 pS/m at the operating temperature involved.This level of conductivity can exist inherently or can be achieved by using a static dissipative additive.Specifically, it is desirable to increase the conductivity of single-phase liquids above 100 pS/m and of liquids containing solids and immiscible above 1,000 pS/m.
An electrostatic discharge can charge or damage new filter elements inside a jet fuel filter separator, even if the fuel has been treated with a conductivity additive.When the filter elements are replaced, the filter-separator should be filled by gravity when possible or the flow velocity should be reduced to 1 m/s until the filter-separator is full of liquid.
The flow and agitation of liquids can also cause electrostatic charging of non-grounded insulating (plastic or rubber) and conductive (metallic) vessels, pipes and fittings.Accumulated charge on insulating (plastic or rubber) and ungrounded metallic devices and equipment can give rise to electrostatic discharges energetic enough to ignite flammable atmospheres.Therefore, it is generally recommended that the agitation of electrically insulating liquids be conducted in an inert atmosphere The build-up of electrostatic charge on the insulating linings (glass or plastic) of vessels and pipes can also lead to the creation of holes in the lining, causing leaks, pipe or vessel corrosion, and liquid contamination.It should be noted that holes can occur even in inert atmospheres.Consequently, it is essential to identify and eliminate or control the generation, accumulation and/or electrostatic discharges.
Grounding of the Liquid.Efforts should be made to keep liquids in continuous contact with electrical ground, even in insulating vessels and plastic-lined piping, in order to minimize the accumulation of electrostatic charge on the liquid.In insulating vessels, a suitable pathway may be provided by a grounded metal bottom runoff valve, a grounded tantalum patch below the liquid surface, or a grounded metal dip pipe.The incidence of pinholing can be eliminated by using an static dissipative or conductive lining.
Limiting the Liquid Velocity.Limiting the liquid velocity during filling operations helps to reduce electrostatic charge generation during pipeline flow, as well as minimizing splashing and spraying in the receiving vessel or container.If flow velocity limitations cannot be observed or if a grounded metal dip pipe cannot be used, it may not be possible to dissipate electrostatic charge from the liquid at a rate sufficient to reduce the probability of ignition from an electrostatic discharge from the liquid to a suitably low level.Therefore, in such cases, inerting.
3 Requirements for limiting the flow rate of liquid fuel when loading car tanks A simple empirical formula for the maximum recommended linear speed to minimize load generation as a function of loading boom diameter has been developed for tank trucks.vd <0.5 m 2 /s Where v is the speed in m/s; d is the inside diameter of the spillway in meters .Flow rates and velocities meeting this limit are shown in Table 2 for selected pipe sizes.In addition, the linear flow velocity must never exceed 7 m/s.These flow restrictions apply to all pipe segments from 0 to 30 seconds (minimum) upstream of the tank fill opening, including the pipe segment on the tank vehicle itself.
The 0.5 m2/s limit does not ensure that static ignition will not occur, but it greatly reduces the probability of ignition.Industrial ULSD loading experience has indicated that the historical limit of 0.5 m2/s may not be adequate for protection.For tankers carrying ULSD (ultra-low sulfur diesel) and diesel the maximum flow rate must never exceed 7 m/s or 0.38 m2/s, whichever is greater.
The main parameter that must be controlled when loading car tanks is the loading flow rate, respectively the speed that must be limited to non-dangerous values corresponding to the conductivity of the fuel.In the first phase of loading, the velocity of the liquid in the filling line should be limited to approximately 1 m/s until the outlet is immersed in the liquid to a height of twice the outlet diameter to prevent splashing/spraying and to minimize surface turbulence.Next, the speed must be limited depending on the conductivity of the fuel and the sulfur content to the maximum values resulting from the tables below.If flow rate limitations cannot be observed or if a grounded metallic dip pipe cannot be used, it may not be possible to dissipate the electrostatic charge from the liquid at a rate sufficient to reduce the probability of ignition from an electrostatic discharge of the liquid at a suitable low level.Therefore, in such cases consideration should be given to inerting the vessel or container before and during filling to minimize the risk of fire and explosion.

The importance of determining the explosivity characteristics
According to the legislation in force, employees must be informed about the dangers of the materials (flammable substances) to which they are exposed when working.
Manufacturers must routinely identify and communicate critical physical and chemical properties and specific precautions before supplying combustible materials The primary method of communicating this information is through Material Safety Data Sheets (MSDS).
If we analyze the datasheets available on the net, it is observed that some suppliers of ULSD indicated that the material can generate an explosive atmosphere and can accumulate a static electrical charge that could discharge and ignite the accumulated vapors.
However, not all MSDS provided critical physical and chemical property data and warnings that the material may form an ignitable vapor-air mixture inside storage tanks and it can be electrostatically charged Nor did they list comprehensive precautions beyond normal bonding and grounding practices, or reference relevant consensus guidance that users could have used to help prevent this explosion.
To prevent explosions with flammable liquids such as ULSD, MSDSs should clearly warn that the material is a static accumulator and may under certain conditions form a flammable vapor-air mixture inside storage tanks and that bonding and grounding should may not be sufficient in some cases.It is also necessary to specify the conductivity of the liquid fuel, so that companies know the degree to which the material will accumulate static and can establish appropriate protective measures.

Conclusions
The importance of knowing the mechanisms by which it is produced is dangerous and the measures that must be implemented to prevent fires and explosions when transporting liquid fuels through pipelines, during their transport and storage in tanks, is emphasized by numerous events such as explosions/fires generated by it, cited in various sources of information, as could be seen from the examples given in the paper.
To prevent these events, an important factor is the updating of the work instructions and the training of the people involved in the design, installation, maintenance and operation of the installations.https://doi.org/10.1051/matecconf/202438900029SESAM 2023 At the same time, the procedures for assessing the risk of explosions and course support must be permanently updated in order to implement them in the activity of training the personnel and respectively of evaluating the installations that are carried out at INSEMEX Petrosani.
In conclusion, it is found that with the appearance of new types of fuel, ULSD (Ultra Low Sulfur Diesel), as a result of the implementation of environmental legislation, the risk of explosion increased, because these types of diesel are easily charged with static electricity and can appear discharges with sufficient energy to ignite an explosive atmosphere.Therefore, oil companies must revise their work procedures with additional measures to prevent explosions.These procedures must implement technical measures to eliminate sources of initiation, which must be complemented by organizational measures.
Attention must be paid to the organizational measures that must be taken by carriers to prevent alternating loading.In this sense, a cooperation agreement between the tanker operator and the site operator is indispensable in order to implement safe loading and unloading procedures, with the aim of mitigating the risk of explosion.

Fig 3 .
Fig 3. Electrostatic charge when liquids flow through pipes

Table 1 .
Safety data sheets of ULSD From the seven safety data sheets of ULSD (table 1), only Tesoro Refining & Marketing Co gives the conductivity of Diesel Low Sulfur (LSD) and Ultra Low Sulfur Diesel (ULSD : https://doi.org/10.1051/matecconf/202438900029SESAM 2023 -Diesel Fuel Oils at terminal load rack: At least 25 pS/m.-Ultra Low Sulfur Diesel (ULSD) without conductivity additive: 0 pS/m to 5 pS/m.